Wireless communication systems—also known as radio communication systems—are well-known in the art. A wireless communication system refers to a communication system having a transmitting end and a receiving end in which signals are transmitted or communicated from the transmitting end to the receiving end via a signal path, wherein a portion of this signal path from the transmitting end to the receiving end includes signal transmission via a wireless interface. This wireless interface is also known as radio interface. Therefore, in wireless communication systems, data (for example voice data, image data or other digital data) is transmitted by means of electro-magnetic waves via this wireless interface.
Hereinafter, the present invention and the underlying problem is described with regard to such wireless communication systems, whereas, it should be noted, that the present invention is not restricted to a wireless communication system, but can also be used for wired communication, such as a broad band-communication.
Radio communication systems are facing constantly increasing challenges. On the one hand, there is the challenge of a limited spectrum which is used for carrying an increasing amount of data to be transmitted. On the other hand, the complicated environment of the radio transmission and individual situations of the data communication links take place inside one single data communication system. This data communication system typically comprises a fixed set of protocols which are—as a consequence of the above mentioned challenges—non-flexible.
In addition, there is a constant demand to introduce higher layer intelligences into the physical layer signal processing. In any wireless communication system, typically so-called pilot channels are provided for channel estimation, cell identification, phase reference, timing information to the different mobile stations. However, these pilot channels can only be used with a fixed pattern.
In a radio communication system such as a UMTS-system, several antennas are used at one base-station to utilise the spatial diversity. These antennas are weighted with different weighting factors for data transmission on dedicated channels to individual mobile stations. These weighting factors are generally complex numbers, consisting of an amount component and a phase component. A set of weighting factors used simultaneously for a dedicated channel is also known as a weighting vector.
With the Closed-Loop Transmit Diversity method, as it is called, a specific mobile station informs the corresponding base station e.g. by feed-back information in the uplink, how it should specify the weighting factors in order that optimum reception is achieved at the mobile station with the corresponding transmission power at the base station. To be able to supply this feed-back information, the mobile station must have estimates of the attenuation and phase shift on the single propagation paths from the various antennas of the base station to the mobile station. These estimates are in a UMTS communication system normally obtained by using a global pilot signal that is transmitted from the base station to all of the mobile stations within the corresponding cell of the base station. This global pilot system is an overlay of pilot-bit sequences of the individual antennas of the base station, with the pilot bit-sequence varying from antenna to antenna and being characteristic in each case of the transmitting antenna. Because these pilot bit-sequences are permanently specified and, therefore, are known to all mobile stations, a specific mobile station is therefore able to extract the pilot bit sequences from the downlink signal and separate these pilot bit sequences from each other, in order to estimate the channel properties, such as the attenuation, the phase shift for each individual antenna, the cell identification, timing information, etc.
This kind of channel estimation using a global pilot signal is well-known in the art and is described, for example, in US 2004/0233872 A1.
Channel estimation is also known in CDMA communication systems such as a WCDMA system. In a WCDMA communication system there exists a primary common pilot channel and a secondary common pilot channel, whereas, each of these pilot channels are coded by specific spreading codes.
A common challenge for wireless OFDM (OFDM=orthogonal frequency division multiplexing) communication systems is the centre frequency off-set estimation and the channel tracking. For MIMO (MIMO=multi-input multi-output) channel equalisation knowledge about the channel state information (CSI) is an important pre-condition. This channel state-information can be obtained, for example, by measuring the channel using known training symbols within the transmission frame.
In order to measure the phase evolution over time, often a second set of dedicated pilot channels is provided, in order to have a second reference point in time, which additionally allows phase-interpolation and prediction over the entire transmission frame, provided that the system parameter design is well-matched to the transmission scenario.
Besides the above mentioned different applications of using a global pilot signal or the corresponding pilot channels it is a constant demand, especially in wireless communication systems to extend the functionality of a data communication.
The present invention is, therefore, based on the object to better use the existing resources within a communication system especially within a wireless communication system.